1. Field of the Invention
This invention relates to electrical meter devices, and more particularly to two-element, single disc polyphase watthour meters.
2. Description of the Prior Art
Multi-element, single disc polyphase watthour meters have been employed for metering polyphase systems for a long time. In recent years, manufacturers have directed their efforts toward producing multielement meters which provide more accurate readings, are simpler to adjust and which are more compact and of lower cost.
A major problem in single-disc multi-element polyphase meters is the interference between the two elements of such meters. As indicated in AIEE Transaction Paper, No. 55-483, by W. I. Schmidt, which is entitled "The Design of a New 2-Element, Single-Disc Polyphase Meter," made available for printing on June 1, 1955, the interference arises from interactions between eddy currents in the disc and fluxes from the electromagnetic poles of the two electromagnetic elements. The interactions include voltage interference, current interference, and voltage-current interference.
Voltage interference and current interference has been substantially eliminated by arranging the two electromagnetic elements symmetrically with respect to the center of the disc. Generally the elements are mounted on a frame secured to a baseplate so as to be positioned in a pair of parallel spaced planes extending between front and rear areas of the meter and thus perpendicular to the baseplate. The problem of voltage-current interference has been more difficult to eliminate. As proposed in the Transaction Paper referenced above, one solution to the problem of voltage-current interference is obtained through the use of compensating windings on the voltage poles. However, the addition of such compensating windings increases the cost of the meter.
The use of a second electromagnetic element in polyphase meters also complicates the light load and phase balance adjustments required to obtain accurate indications of power usage. The presence of two electromagnetic elements operating on a common disc results in interactions not experienced in single element meters. Thus, efforts have been made to simplify the design of light load and phase balance adjusters.
As indicated above, in known multi-element meters, the two elements are mounted perpendicular to the baseplate. While such mounting arrangement does not necessarily require an increase in the size of the meter, it does generally preclude reduction in the size of the meter which would provide a more compact unit. Also, such mounting arrangement necessitates the use of a mounting plate and frame which are different from the ones used for single element meters, for example. Accordingly, the production costs for multi-element polyphase meters, which are relatively low volume units as compared to single element meters, are higher than those for single element meters, particularly due to the need for two or more castings and the use of non-standard parts, that is, parts not used in single element meters.
A further consideration is that the electrical power systems in which watthour meters are used may have a number of ratings, and the watthour meters, which may be either single or multi-element meters, are required to register power usage in systems where the voltage may be 120 VAC or 240 VAC, and the nominal current may be 2.5, 15 or 30 amps. This requires production of five different registers, having different gear trains, to permit the dial indicators to provide the correct reading for a given rating.
It would be desireable to have a multi-element polyphase watthour meter in which voltage-current interference is minimized without the need for an additional compensating winding. It would also be desireable to have a polyphase watthour meter having improved accuracy and simplified balance adjustments, and which is more compact and of lower cost than known multi-element meters. It would further be desireable to have a watthour meter which is readily adaptable for use in measuring power usage for power systems of different power ratings.